1. Field of the Invention
The invention relates generally to microwave equipment and more specifically to using capacitive T-networks implemented with thin film methods.
2. Description of the Prior Art
Networks of capacitors and inductors can be used in various microwave applications to make bandpass filters. FIG. 1A shows a typical capacitor pi-network (so-called because its configuration resembles the Greek letter .pi.) in narrow bandwidth filters. A convenient measure of bandwidth is the percentage bandwidth (% BW), which is: ##EQU1## where f.sub.0 is the center frequency, f.sub.1 is the lower cutoff frequency, and f.sub.2 is the upper cutoff frequency.
A typical implementation of such a pi-network 10 is shown in FIGS. 1B and 1C. Pi-network 10 comprises metal plates 12, 14, and 16 separated by a polyamide layer 18 on an alumina substrate 20 having a groundplane 22. Capacitor C1 is formed between metal plate 12 and groundplane 22. Capacitor C3 is formed between metal plate 14 and groundplane 22. Capacitor C2 is actually two capacitors formed by metal plates 12 and 16, for one of the series capacitors, and metal plates 16 and 14, for the other. Such a network is described in U.S. Pat. No. 4,881,050, issued Nov. 14, 1989 to Swanson, Jr. There, a filter is fabricated from a planar dielectric substrate having a ground plane on one side and two thin-film metal layers and an insulation layer on the other side. The metal and insulation layers are configured to form one or more capacitive pi-networks and spiral inductors, which are electrically interconnected to form the filter.
In order to increase the % BW, capacitors C1 and C3 must be made smaller and capacitor C2 must be made bigger. The sizes of the plate areas, dielectric thickness, and dielectric materials used can all be adjusted to manipulate the values of the capacitors, but practical limits are reached at even modest %BW figures. The problem, traditionally, with microwave filters has been the very limited dynamic range of element values. (Which is not a problem at lower frequencies because discrete capacitors and inductors can be used.) The prior art solutions have usually centered on coming up with clever topologies to get around those restrictions.